This invention relates to an image forming apparatus having applications as color printing apparatuses.
Various color printing apparatus are known and adapted to form an image by laying toner images of different colors one on the other. The image may be formed on a photosensitive body, a sheet of paper, on an intermediate transfer body or some other recording medium.
The method of laying toner images one on the other on a photosensitive body is accompanied by a disadvantage of poor image quality because the toner images have to be exposed to light.
The method of laying toner images one on the other on a sheet of paper has been popularly used for long. This transfer method provides an advantage that the toner images are transferred only once as compared with the intermediate transfer method by which the toner images need to be transferred twice. Hence, this method has been employed in various color copying machines.
On the other hand, the intermediate transfer method that has been regarded as disadvantageous in terms of image quality has been employed in color printers that do not required a very high image quality because it provides an advantage of requiring only a small space if the intermediate transfer body is realized in the form of a belt and that of applicability of cardboard.
The intermediate transfer body may be realized in the form of a belt or a drum.
An intermediate transfer body in the form of a drum (to be referred to as intermediate transfer drum hereinafter) is typically prepared by coating the surface of a base metal pipe with a rubber belt.
Problems occur when the rubber belt slides on the base pipe. The problems include relative displacements of different colors and defective or uneven transfers of toner images due to a warped and lifted rubber belt. The rubber belt can be warped mainly due to the load torque at the primary transfer nip of the intermediate transfer drum and the photosensitive body, the contact load of the secondary transfer roller in the secondary transfer process and/or the load torque of the cleaner contacting the intermediate transfer drum.
The system where the intermediate transfer drum is not equipped with a dedicated drive source and adapted to follow the rotary movement of the photosensitive body provides an advantage of scarcely producing color displacements because both the photosensitive body and the intermediate transfer drum revolve substantially at a same rate.
However, with this system, the surface of the intermediate transfer drum is subjected to a large twisting force at the primary transfer nip when the photosensitive body or the intermediate transfer drum starts rotating. If such a condition repeatedly appears, the rubber belt can be lifted from the base pipe to give rise to a problem of defective transfer.
If the intermediate transfer drum is equipped with a dedicated drive source, the rubber belt can become lifted from the base pipe to give rise to a problem of defective transfer as a result of repeated use when the photosensitive body and the intermediate transfer drum revolve at respective rates that are different from each other when they start rotating.
In printing apparatuses where a blade cleaner is held in contact with the secondary transfer roller, the surface of the intermediate transfer drum can be subjected to a large twisting force when the load torque of secondary transfer roller is large and the secondary transfer roller is made to contact the intermediate transfer drum.
When a blade is used as a cleaner for the intermediate transfer drum, the surface of the intermediate transfer drum can be subjected to a large twisting force to lift the rubber belt due to the load torque of the blade.
Therefore, when coating the base pipe with a rubber belt, a technique of applying an adhesive agent between the base pipe and the rubber belt is usually used in order to prevent the rubber belt from sliding on the base pipe.
However, unless the adhesive agent is uniformly applied between the base pipe and the rubber belt, the surface of the intermediate transfer drum shows undulations to give rise to uneven transfers, which prevents efforts for improving the yield and realizing mass production.
Uniform transfers will be realized to improve the yield if the base pipe can be coated with a rubber belt without any adhesive agent. Then, the elastic rubber belt needs to be expanded maximally before the base pipe is coated with it in order to prevent the rubber belt from sliding on the surface of the base pipe without using any adhesive agent.
However, if the rubber belt is expanded excessively, its elasticity will become lost as it is used for a long time. Then, it will slide on the base pipe. Additionally, expanding the highly elastic rubber belt excessively to produce an intermediate transfer body will constitute a manufacturing problem.
In view of the above identified circumstances, it is therefore the object of the present invention to provide an image forming apparatus in which the elastic belt will not slide on the base pipe if the base pipe is coated with the elastic belt by expanding the latter excessively.
In an aspect of the invention, the above object is achieved by providing an image forming apparatus comprising an image forming device which forms an image of a developing agent on an image carrier; a freely rotatable intermediate transfer body to which the developing agent image formed by the image forming device is to be transferred and a freely rotatable transfer roller which transfers the developing agent image from the intermediate transfer body further to a final target of transfer, the intermediate transfer body comprising a base pipe and an elastic belt, the base pipe being coated with the elastic belt, the image carrier and the intermediate transfer body being driven to rotate by a same drive source.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.